312 MANUAL OF NATURAL HISTORY. 



drawn into its shell-like habitation. Some of the 

 Wandering Annelids, as the Sea-Centipedes, swim 

 readily along the surface of the water, and are 

 vividly phosphorescent during the night; most of 

 them, however, live in damp sands along the coast. 

 A few among them attain to considerable dimen- 

 sions, specimens of Leodicea gigantea being known 

 twelve feet in length. The sides of many are covered 

 with beautiful iridescent setae arranged in tufts ; 

 these bristles are often converted into barbs and 

 harpoons, which, however, are enclosed in horny 

 sheaths, into which these sharp weapons of defence 

 can be retracted, and thus avoid injuring their pos- 

 sessor ; these are well seen in the Aphrodite his- 

 pida. Sometimes the bristles are placed on tubercles, 

 and sometimes they are reduced to a few stiff hairs, 

 as in the Earth-worm. In Nereis prolifera re- 

 production occurs by means of spontaneous division, 

 the hind part of the body becoming provided with 

 a new head, and the fore part with a new tail. 



In this Class is found the invaluable medicinal 

 Leech (Hirudo), and its consimilar, the Horse- 

 Leech (Sanguisuga). The species of Leeches em- 

 ployed as medicinal agents, are the Hirudo medici- 

 nalis and H. officinalis, which are principally ob- 

 tained from the South of France, Sweden, Poland, 

 and Hungary. The cupping apparatus consists of 

 the sucker surrounding the mouth and three jaws 

 with serrated edges placed in a triangle which act 

 the part of scarificators, and which occasion the 

 tri-radiate form of the bite. Of such value is this 



